Clones of cultured mouse myeloma cells which are variant in their ability to synthesize, assemble, and secrete Ig will be isolated. Variants will be sought which: 1) have lost the ability to synthesize one of the three polypeptide chains, 2) produce chains with altered primary structure, 3) are altered in their ability to bind antigen, 4) assemble Ig at a different rate or via an altered pathway, 5) are altered in their ability to secrete their Ig, or 6) have altered surface Ig. These variants will be characterized with respect to the frequency of occurrence and the structural and cellular changes involved. Isolation and characterization of these variants should give information about control and regulation of Ig producing cells, the structure-function relationships in the Ig molecule, the genetic organization of the Ig locus, and cellular functions in Ig biosynthesis. Detailed molecular characterization should define the precise mechanisms of variation. Secretion variants clarify the necessary requirements for the secretion of a cellular protein. Antigen binding variants should give insight into the feasibility of the somatic generation of antibody diversity.